wm: Remove hard gtkmm dependency from SConstruct.
wm: Remove hard gtkmm dependency from SConstruct.

This was just there for mock_chrome, which we don't
build within the chroot.  I tried to move mock_chrome
into a tools/ subdirectory with a SConstruct that
sources window_manager/SConstruct, but it ended up
getting tricky because of relative include paths.
Instead, I'm just making window_manager/SConstruct
conditionally declare the mock_chrome target
depending on whether gtkmm is present.  Let me know
if you think this is too hacky.

(The next change after this will pare down the
dependencies in the window manager's ebuild file
pretty significantly.)

BUG=none
TEST=built it

Review URL: http://codereview.chromium.org/668206

joshfire/chromiumos-platform-window_manager,joshfire/chromiumos-platform-window_manager,joshfire/chromiumos-platform-window_manager,joshfire/chromiumos-platform-window_manager